It's a Great mitzvah to be happy

BH today the Halacha is like Rabbainu Rabbi Nachman of Breslov better known as Na NaCh NaChMa NaChMaN MAyUMaN who said it is a great mitzvah to be happy always, but in previous good times this wasn't so simple, as we see in the Talmud:

Mar, the son of Rabina, made a marriage banquet for his son; when he observed that the Rabbis were at the height of their merriment he brought in a very costly cup worth four hundred zouzim and broke it before them; and they grew sad. R. Ashi made a marriage feast for his son and when he noticed the Rabbis were at the height of their merriment (Fol. 31a) he brought in a costly cup made of white glass and broke it before them; and they grew sad. At the wedding of Mar the son of Rabina, the Rabbis said unto R. Hammma Zuti: "Sing to us master." He sang: "Woe, for we must die! Woe, for we must die!" "And what shall we respond to it?" asked they. "Sing ye," answered he, "Alas! where is the Torah [we have studied] and where are the meritorious deeds? [we have performed] to protect us?" R. Jochanan in the name of R. Simon b. Jochai said: "It is not permitted to a man to fill his mouth with laughter in this world, for it is said (Ps. 126, 2.) Then shall our mouths be filled with laughter, and our tongue with singing — when shall this be? At the time when — they shall say among the nations 'great things hath the Lord done for these.'" It is related of Resh Lakish that he never laughed from the time he heard this from R. Jochanan, his teacher.

Ein Yaakov, Berachot 5

Thank God for Na NaCh NaChMa NaChMaN MAyUMaN